1. Field of the Invention
This invention relates to a cold shield for use in an infrared optical system and, more particularly, to the use of optical fibers or the use of a thin film angle filter to function as a cold shield in an infrared optical imaging system such as an electro-optic seeker system.
2. Description of Related Art
Infrared optical systems using high performance semiconductor detector materials usually require cryogenic cooling in order to increase detector sensitivity. The detectors are typically cooled to a temperature of approximately 77.degree. Kelvin by liquid nitrogen. In these systems, optimum performance is achieved by "cold shielding" the detector array by introducing a cold diaphragm in front of the detector array so that the viewing angle of the detector to the warm background is limited as far as possible to that required for transmission of the radiation from the scene. The cold diaphragm is typically cooled to a temperature (e.g., 80.degree. Kelvin) close to that of the detector. Since the radiation reaching the detector array from outside this viewing angle is emitted from the cold diaphragm, it is generally negligible so that the condition of minimum background radiation, and, hence, minimum background noise, is achieved. In other words, cold shielding of a detector array is typically achieved with a cryogenically cooled diaphragm or aperture stop in front of the cryogenically cooled detector array. The cold shield diaphragm and detector are placed within a Dewar, a vessel in which they are cryogenically cooled.
FIG. 1 presents a seeker configuration known to the assignee of the present application. The seeker configuration includes a dome 10 and a three element optical system 7 comprising three lenses 11, 13, and 15 particularly adapted for transmitting and focussing infrared radiation. The seeker configuration further includes a Dewar window 18 and an aperture stop 19. The aperture stop 19 acts as the cold shield in this embodiment. A cold filter 21 is positioned between the aperture stop 19 and the focal plane 22. A solid state infrared detector array (not shown) is typically positioned at the focal plane 22. This seeker configuration provides approximately 100 percent cold shielding with a moderate aperture.
Note that, in the seeker configuration of FIG. 1, the optical aperture is smaller than the aperture A of the first imaging element of the optical system after the dome 10. Therefore, the seeker configuration is not optimal.
In one application, the dome 10 forms the nose of a guided projectile or missile. However, the system of FIG. 1 is too large for smaller projectile/missile systems. The aperture may be reduced by utilizing an optical configuration which incorporates optical relay elements. Optical relay elements tend to place the entrance pupil at the front element when the system aperture is at the cold shield location in the Dewar. Unfortunately, this system is too long and complex to be practical for use in small missiles or guided projectiles.
Therefore, it is desirable to provide an electro-optic infrared seeker system with high sensitivity while maintaining a small package size. Two system parameters must be optimized to provide this goal; 1) optical aperture and 2) efficient detector cold shielding or cold stop. Prior to the present invention, it was necessary to accept undesirable compromises in sensor performance by reducing the required size of the optical system.